1. Field of the Invention
The present invention relates to a steer-by-wire steering device which is mounted in a vehicle, and more particularly to a control means for an actuator which applies an external force for force feedback to a steering operation element such as a steering wheel.
2. Description of the Related Art
One example of a steer-by-wire steering device which has been proposed conventionally is explained hereinafter in conjunction with FIG. 9 and FIG. 10. FIG. 9 is a view showing the constitution of the steer-by-wire steering device which has been proposed conventionally and FIG. 10 is a graph showing the relationship between an external force applied to a tire and an external force acting on a steering operation element.
As shown in FIG. 9, the steer-by-wire steering device of this example includes a steering wheel 21 which constitutes a steering operation element, an operating angle sensor 22 which detects an operation angle of the steering wheel 21, an operating element actuator 23 which applies an external force for force feedback to the steering wheel 21, a tire reactive force sensor 24 which constitutes a detection means for detecting a travelling condition of a vehicle, a steering actuator 25 which applies an external force to a steering system of the vehicle, and a controller 26 which receives output signals of the operating angle sensor 22 and the tire reactive force sensor 24 and controls the driving of the operating element actuator 23 and the steering actuator 25.
In a mechanical steering device, a tire and a steering wheel are connected to each other by way of a mechanical link mechanism and an external force which is applied to the tire from a road surface during travelling is directly transmitted to the steering wheel. Accordingly, the external force applied to the tire and the external force applied to the steering wheel exhibit the proportional relationship as shown in FIG. 10.
In the conventional steer-by-wire steering device, to obtain an operation feeling similar to that of the mechanical steering device, the controller 26 controls the driving of the operating element actuator 23 in response to signals inputted from the tire reactive force sensor 24 so as to apply the external force which is proportional to the external force applied to the tire to the steering wheel.
With the provision of the above-mentioned conventional steer-by-wire steering device, the operation feeling similar to that of the mechanical steering device can be obtained.
However, since this steer-by-wire steering device adopts the constitution in which the external force which is proportional to the external force applied to the tire is applied to the steering wheel 21, when a sudden and large external force acts on the tire, for example, when the tire gets over a shoulder of a road or when the vehicle enters an unpaved road or a dirt road from a paved road or when the vehicle receives a cross wind at an exit of a tunnel, a large external force acts on the steering wheel 21 so it is feared that the travelling of the vehicle becomes unstable. Such a drawback is particularly apparent when the vehicle speed is high, a torque which acts on a drive shaft of the vehicle is high, when the operation acceleration of the steering wheel is high, when a gripping force of the tire is lowered due to a low ambient temperature, and when a surface of road is wet so that a gripping force of the tire is lowered or the like.
Further, in the above-mentioned conventional steer-by-wire steering device, when the external force which acts on the tire is small, the external force which acts on the steering wheel 21 is made small proportionally. For example, when the external force acts on the tire is extremely small as in a case in which a vehicle performs straight travelling at a high speed on an express way, the external force applied to the steering wheel 21 by the operating element actuator 23 also becomes extremely small so that even when a driver applies an extremely small operation force to the steering wheel 21, there may arise a drawback that the vehicle largely staggers or meanders
The present invention has been made to solve these drawbacks of the prior art and it is an object of the present invention to provide a steer-by-wire steering device which is capable of applying an optimum external force to a steering operation element in response to a travelling condition of a vehicle or a condition of a road surface.
To solve the above-mentioned drawbacks, according to the present invention, there is provided a steer-by-wire steering device comprising a steering operation element, an actuator which applies an external force to the steering operation element, a controller of the actuator, and detection means for detecting a travelling condition of a vehicle including at least a tire reactive force sensor which detects an external force applied to a tire of a vehicle from a road surface, wherein the controller stores a plurality of functions which use a detection signal inputted from the detection means as a parameter therein, the controller selects an optimum function from the plurality of functions in response to a level of the detection signal inputted from the detection means, and the controller computes a control signal for the actuator based on the selected function and a level of the detection signal and outputs the control signal.
In the above-mentioned steer-by-wire steering device, as the detection means, in addition to the tire reactive force sensor, the steering device is further provided with at least one type of sensor selected from a group of sensors including a vehicle speed sensor which detects a travelling speed of the vehicle, a torque sensor which detects torque acting on a drive shaft of the vehicle, an acceleration sensor which detects operation acceleration of the steering operation element, a temperature sensor which detects an ambient temperature, and an image pick-up sensor which detects a road surface condition.
In the above-mentioned steer-by-wire steering device, the function which is selected when the level of the detection signal is equal to or below a first prescribed value is a function which sets the external force applied to the steering operation element to an external force smaller than an external force proportional to the level of the detection signal, the function which is selected when the level of the detection signal is equal to or above a second prescribed value is a function which sets the external force applied to the steering operation element to an external force larger than an external force proportional to the level of the detection signal, and the function which is selected when the level of the detection signal is equal to or below the first prescribed value and is equal to or above the second prescribed value is a function which sets the external force applied to the steering operation element to an external force proportional to the level of the detection signal.
In the above-mentioned steer-by-wire steering device, the function which is selected when the level of the detection signal is equal to or below a first prescribed value is a function which sets the external force applied to the steering operation element to an external force larger than an external force proportional to the level of the detection signal, the function which is selected when the level of the detection signal is equal to or above a second prescribed value is a function which sets the external force applied to the steering operation element to an external force smaller than an external force proportional to the level of the detection signal, and the function which is selected when the level of the detection signal is equal to or below the first prescribed value and is equal to or above the second prescribed value is a function which sets the external force applied to the steering operation element to an external force proportional to the level of the detection signal.
In the above-mentioned steer-by-wire steering device, provided that the image pick-up sensor which detects the road surface condition is provided as the detection means, the function which is selected by the image pick-up sensor when the road surface is wet is a function which sets the external force applied to the steering operation element to an external force larger than an external force proportional to the level of the detection signal, and the function which is selected by the image pick-up sensor when the road surface is dry is a function which sets the external force applied to the steering operation element to an external force smaller than the external force proportional to the level of the detection signal.
In the above-mentioned steer-by-wire steering device, provided that the acceleration sensor which detects operation acceleration of the steering operation element is provided as the detection means,
the function which is selected when operation acceleration detected by the acceleration sensor is equal to or more than prescribed acceleration is a function which sets the external force applied to the steering operation element to an external force smaller than an external force proportional to the level of the detection signal.
In this manner, the controller preliminarily stores a plurality of functions which use a detection signal inputted by detection means as a parameter therein. Then, the controller selects an optimum function from the plurality of functions which are preliminarily stored in response to the level of the detection signal inputted from the detection means. Thereafter, the controller computes the control signal for the actuator based on the selected function and the level of the inputted detection signal and outputs the control signal. As a result, the controller can perform control such that, for example, when a sudden and large external force acts on the tire, the external force applied to the steering operation element is made relatively small, while when the external force which acts on the tire is small, the external force applied to the steering operation element is made relatively large. Accordingly, it is possible to apply an optimum external force which corresponds to the travelling condition of the vehicle and the road surface condition to the steering operation element whereby the operation performance and the reliability of the steer-by-wire steering device can be enhanced.